One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series or in parallel with each other so as to construct a medium- or large-sized battery pack.
However, a large amount of heat is generated from the medium- or large-sized battery pack during the charge and the discharge of the battery pack. Most of the heat is generated from the secondary batteries, which are the unit cells of the medium- or large-sized battery pack. When the heat generated from the unit cells during the charge and the discharge of the battery pack is not effectively removed, heat is accumulated in the battery pack with the result that the unit cells are degraded. According to circumstances, the unit cells may catch fire or explode. Consequently, it is necessary to provide a cooling system for preventing the catching fire or explosion of the unit cells.
The cooling system is constructed in a structure in which a coolant is forcibly circulated through the interior of the battery pack so as to remove heat from the unit cells, i.e., in a contact type cooling structure in which the coolant is brought into contact with the surfaces of the unit cells constituting the battery pack. Air is mainly used as the coolant. Consequently, the cooling system is generally constructed in a contact type air cooling structure.
Meanwhile, prismatic batteries or pouch-shaped batteries, which can be stacked one on another to reduce the size of a dead space, are used as the unit cells. In order to easily accomplish the mechanical coupling and the electrical connection between the unit cells, a cartridge, in which one or more unit cells are mounted, is used. And a plurality of cartridges, in which the unit cells are mounted, are stacked one on another so construct a battery pack.
The cartridge may have various shapes. In addition, the cartridge may be constructed in a structure in which the unit cells are fixed to a frame member while most of the outer surfaces of the unit cells are open. An example of such a cartridge is disclosed in Korean Patent Application No. 2004-111699, which has been filed in the name of the applicant of the present patent application. FIG. 1 illustrates the cartridge disclosed in the above-mentioned application.
Referring first to FIG. 1, a cartridge 100 comprises a pair of frame members 120 and 122, which are coupled with each other. Unit cells 200 and 201 are located in cell partitions 130 of the frame members 120 and 122 while the frame members 120 and 122 are separated from each other, and are then securely fixed at the cell partitions 130 of the frame members 120 and 122 after the frame members 120 and 122 are coupled with each other. The unit cell 200 has an electrode lead (not shown), which is electrically connected to that of the neighboring unit cell 201 via a bus bar 140 located at the upper part of the cartridge 100. As shown in FIG. 1, the unit cells 200 and 201 are connected in series with each other. According to circumstances, however, the unit cells may be connected in parallel with each other. The unit cells are electrically connected to a cathode terminal 150 and an anode terminal 160, which protrude from opposite sides of the upper end of the cartridge 100, respectively.
FIG. 2 is a perspective view illustrating a battery module constructed by stacking a plurality of battery cartridges one on another in an alternating orientation structure.
Referring to FIG. 2, a plurality of cartridges 101, 102, 103 . . . are stacked one on another in the thickness direction so as to construct a battery module 300. To easily accomplish the electrical connection between the terminals of the cartridges, the second cartridge 102 is stacked on the first cartridge 101 while the second cartridge 102 is oriented in the direction opposite to the orientation direction of the first cartridge 101. Specifically, the first cartridge 101 and the second cartridge 102 are arranged such that a cathode terminal 152 and an anode terminal 162 of the second cartridge 102 are opposite to a cathode terminal 151 and an anode terminal 161 of the first cartridge 101. Similarly, the second cartridge 102 and the third cartridge 103 are arranged such that a cathode terminal 153 and an anode terminal 163 of the third cartridge 103 are opposite to the cathode terminal 152 and the anode terminal 162 of the second cartridge 102. That is to say, the third cartridge 103 is arranged in the same orientation as the first cartridge 101.
As shown in FIG. 1, the height of an upper end frame 110 and a lower end frame 112 of the cartridge 100 is less than that of side frames 11 of the cartridge. Consequently, when the cartridges 101, 102, 103 . . . are stacked one on another as shown in FIG. 2, flow channels 170, 171, 172, and 173 are formed in spaces defined between the upper ends of the cartridges 101, 102, 103 . . . and the lower ends of the neighboring cartridges. As a result, a coolant flows in the direction indicated by an arrow.
FIG. 3 is a typical view illustrating the flow of a coolant through the flow channel defined between the battery cartridges of the battery module shown in FIG. 2, and FIG. 4 is an enlarged view illustrating part A of FIG. 3.
Referring to FIG. 3, the flow channel 170, which was described above with reference to FIG. 2, is formed between the first cartridge 101 and the second cartridge 102. The respective cartridges 101 and 102 are constructed in a frame structure in which the outer surfaces of the unit cells are almost fully exposed as shown in FIG. 1. As a result, the coolant flowing through the flow channel 170 is brought into direct contact with the outer surfaces of the unit cells (not shown). Consequently, the coolant takes heat generated from the unit cells, while the coolant flows through the flow channel 170, and then discharges the heat out of the battery module.
As shown in FIG. 4, however, when viewing from a microscopic viewpoint, the coolant flowing through the flow channel 170 has a velocity gradient in the hydrodynamic aspect. Specifically, the flow velocity of the coolant flowing through the flow channel while being near to the surface of the unit cell 200 is less than that of the coolant flowing through the flow channel while being spaced apart from the surface of the unit cell 200. This velocity gradient is a factor that greatly reduces the efficiency when heat is removed from the outer surface of the unit cell 200, i.e., the heat removing efficiency.
Consequently, the necessity of a technology for improving the cooling efficiency by solving the velocity gradient is very high in consideration of the fact that the coolant-circulating cooling system is principally based on a contact type cooling reaction mechanism.
One of methods of improving the cooling efficiency may be a method of increasing the flow speed of the coolant to increase the flow rate of the coolant adjacent to the unit cells per hour. However, this method is not preferable in that larger cooling fan or stronger fan driving unit is necessary.